Efforts are on-going to develop and standardize communications networks and protocols intended to meet the requirements set out for the fifth generation (5G) of wireless systems, as defined by the Next Generation Mobile Networks Alliance. Such networks are expected to support a large number of use cases, with different use cases having widely different requirements in terms of the service provided by the network.
For example, some use cases may require that data be transmitted and received with extremely low latency, whereas other use cases may have more relaxed latency requirements. In the former category, it is envisaged that future networks may allow for the remote control of machinery, or surgical instruments. In such cases, it is important that data transmitted between the controller (e.g. a surgeon) and the controlled device (e.g., surgical instruments) is reliable and has low latency. A class of communications requiring such performance has been defined as “ultra-reliable and low-latency communications” (URLLC). See, “Study on New Radio Access Technology; Radio Interface Protocol Aspects” (3GPP TR 38.804, v0.4.0). Note that URLLC traffic is applicable in a wide range of use cases not limited to the surgical/machinery examples set out above. Other communications requiring low latency may be critical machine-type communications (C-MTC). Conversely, in the latter category, large-scale sensor networks and other reporting mechanisms for wireless devices may have no need for low latency. For example, massive machine-type communications (M-MTC) may fall within this category.
Thus, in the present Long Term Evolution (LTE) system and also in future systems, there are many different types of services with different corresponding quality of service (QoS). Such services are typically mapped to corresponding logical channels and each logical channel is associated with a preconfigured logical channel priority (LCP). According to the LCP values, a scheduler in the radio access network (RAN) can flexibly allocate the resources to different logical channels in accordance with the LCP values (e.g., allocating resources to logical channels with higher priority before allocating resources to logical channels with lower priority). In this way, high-latency services may be multiplexed with other less latency-dependent services.
Current versions of LTE are based on a repeated frame structure in which a frame comprises 10 subframes, each of 1 ms length and consisting of 14 orthogonal frequency-division multiplexed (OFDM) symbols. In downlink (DL), the first four symbols or fewer in each subframe comprise a control channel (i.e. the physical downlink control channel, PDCCH), while the remaining symbols comprise a data channel (i.e. the physical downlink shared channel, PDSCH). In uplink (UL), all symbols can be used for the transmission of data (i.e. via the physical uplink shared channel, PUSCH), while some symbols may be used for control information (i.e. via the physical uplink control channel, PUCCH) and reference symbols.
In LTE, scheduling and transmission are defined on the timescale of subframes. That is, terminal devices are scheduled to transmit or receive messages using radio resources that are defined in terms of whole subframes. This timescale is often referred to as the transmission time interval (TTI), i.e. the duration of a transmission on the radio link. Thus the standard TTI in LTE is one subframe, or 14 OFDM symbols.
The current solutions for achieving low latency in LTE rely on the LCP values associated with logical channels. However, transmissions are still limited to TTIs which are 14 symbols long.
A method of reducing this latency still further is desirable, particularly for classes of data requiring extremely low latency.